The present invention relates to a non-linear element and, in particular, to non-linear elements utilized in driving liquid crystal displays.
Reference is first made to FIGS. 1 and 2 wherein a conventional non-linear element, generally shown by the cross hatching of FIG. 1 and in detail in FIG. 2, for driving the picture element of a liquid crystal display device, is provided. A first electric conductor 1, an insulator 3 and a second electric conductor 2 are laminated together, forming the non-linear element generally indicated at 7. A picture element electrode 5 is supported on a substrate 6. First electric conductor 1 is connected to picture element electrode 5 through second electric conductor 2. First electric conductor 1 is made of tantalum (Ta), insulator 3 is made of tantalum oxide (TaO.sub.x), electric conductor 2 is made of chromium (Cr) and picture element electrode 5 is made of a transparent conductive film of ITO. Each of the film is formed on substrate 6 which is made of glass.
A liquid crystal display device generally includes a glass substrate upon which a picture element electrode, a non-linear element for applying an electric charge to the picture element electrode and a wiring pattern for connecting adjacent picture elements are formed. A glass substrate disposed at a space interval of several microns has a pattern formed thereon to act as a counter electrode. A liquid crystal is sealed and oriented between the two glass substrates. In an exemplary embodiment, the two substrates are transparent to enhance their function as a display device for transmitting the light to the liquid crystal and controlling the amount of transmitted light.
A fluorescent light source is utilized. To control the amount of transmitted light, the fluorescent light is first linearly polarized by a polarizing plate. The polarized light enters the liquid crystal display device and the polarizing direction of the polarized light is changed by changing the orientation of the liquid crystal. The light is then transmitted through a polarizing plate changing the amount of light transmitted through the polarizing plate to achieve a display. Then, an appropriate electric potential is applied to the liquid crystal at the region wherein the orientation of the liquid crystal is to be changed, thus changing the orientation of the liquid crystal material. The minimum region to be changed is a picture element. Because the orientation of the liquid crystal can be changed by applying an electric potential, the amount of transmitted light can be controlled by the electric potential applied to the picture element.
In the prior art, each picture element comprises a non-linear element to maintain the electric potential once it has been applied until another potential is applied to the element. The ideal properties for such an element are that the resistance of the element becomes zero when the potential of the non-linear element is applied to the picture element and the resistance becomes infinity once the electric potential is being maintained.
Generally, when a non-linear element is utilized to drive a liquid crystal display, the area of the element should be small enough in comparison with the electrode area of the transparent picture element to obtain the above ideal element properties as near as possible. If the electrode area of the transparent picture element is 200.times.200.mu.m.sup.2, the non-linear element is designed to have an area of 4.times.4.mu.m.sup.2 or less. When determining the size for a non-linear element, various parameters must be taken into consideration. One of the most important parameters is the respective ratios of the capacitance or resistance of the transparent picture element electrode and the non-linear element. Another parameter of concern is the processing capability for manufacturing the non-linear element. Generally, the ratio of the area of the electrode of the transparent picture element to that of the non-linear element is set within the range of 1:1500 to 1:3000. If the electric potential difference between adjacent picture elements is several percent, it can be confirmed by eye measurement. Consequently, to obtain the same level of uniformity between the areas of the non-linear elements, each of the non-linear elements must have an area within 1/10.mu.m.sup.2.
The conventional non-linear elements have been satisfactory. However, they suffer from the fact that because the non-linear elements are extremely fine, it is difficult to form a large number of non-linear elements as required to drive 100,000 picture elements or more evenly over a wide area because dispersion occurs during the photo etching used as a method for forming the non-linear elements. It has also been difficult to improve the yield of the liquid crystal display which includes integrated non-linear elements.
Another disadvantage is that electric conductor 2 does not always adhere closely to substrate 6 resulting in electric conductor 2 separating from substrate 6 or breaking on substrate 6 decreasing the output of the liquid crystal display device which includes these integrated non-linear elements. In particular, when first electric conductor 1 is made of tantalum, second electric conductor 2 is made of chromium, substrate 6 is made of glass and dry etching is performed on the tantalum of first electric conductor 1 to form the non-linear element, the adhesion between the chromium of the second electric conductor 2 and the glass of substrate 6 is poor. This results in chromium separating from substrate 6 or wearing off of substrate 6. The defects in second electric conductor 2 in the vicinity of the stepped portion of first electric conductor 1 becomes quite great.
When forming non-linear element 7, it is preferred that a pattern be formed on first electric conductor 1 by dry etching. If wet etching were to be applied, the stepped portion at the end face of the pattern of first electric conductor 1 becomes too steep so that second electric conductor 2 breaks at the step portion without providing step coverage. A taper is formed at the step portion at the pattern end face of first electric conductor 1 through dry etching to improve the step coverage for second electric conductor 2. However, during formation, substrate 6 is exposed to plasma utilized during dry etching after the first electric conductor is etched. This results in a rough substrate surface. This roughness is even more pronounced in glasses containing many impurities, such as inexpensive glass. No discernable roughness is found in crystal glass such as quartz glass. When the substrate surface becomes rough, the roughened surface increases the occurrence of electric conductor 2 falling off of substrate 6 or wearing off of substrate 6.
Additionally, tantalum or silicon which are etched by dry etching are sometimes redeposited on the substrate as carbon polymers, which cannot be removed by the plasma utilized during the etching. The carbon polymer deposited on the substrate often forms a belt on the substrate at a position several micrometers from the step portion of the pattern and face of the first electric conductor. This results in a decreased adhesion of the electric conductor formed at the belt resulting in the wearing off or falling off of second electric conductor 2 as seen in FIGS. 3 and 4. The defects in the pattern of second electrode conductor 2 occur at a rate of several percent to tens of percent, thereby greatly deteriorating the output of non-linear element 7.
Accordingly, it is desirable to provide a non-linear element which overcomes the shortcomings of the prior art devices described above.